Flux control for induction heating of melt-out cores

ABSTRACT

An induction heating system for melting metal cores contained within articles or components made of plastic uses induction heating and redirects flux lines for efficient melting. The system includes a high permeability magnetic core, an induction heating coil about at least a portion of the magnetic core to produce flux circulation in a magnetic circuit formed by the magnetic core, and two magnetic nozzles attached to the magnetic core with a gap therebetween for insertion of a specific shape of plastic article, the magnetic nozzles shaped to direct and position a desired pattern of flux lines to heat and melt the metal core of the plastic article.

Cross-Reference to Related Applications

This is a division of application Ser. No. 268,552, filed Nov. 8, 1988,now U.S. Pat. No. 4,952,346.

FIELD OF THE INVENTION

The present invention relates to the melting of metal cores containedwithin articles or components made of plastic by means of inductionheating. More specifically, the present invention provides for thepositioning or redirecting of flux lines for efficient melting of metalcores around which plastic articles or components have been molded.

DESCRIPTION OF THE RELATED ART

Fusible metal cores having a complex shape provide a detailed internalconfiguration for a molded plastic article or components. Thesecomponents are used in the automobile industry and other industries inplace of metal components and to avoid the necessity of machining. Themetal cores are made of a low melting point alloy that are subsequentlyremoved from the work piece by melting. The plastic material of thearticle or component is not damaged by or deformed when heated to themelting temperature of the metal alloy for melting the metal cores.

Induction heating coils have been used for melting metal cores. However,in the past, the coils have to be designed and manufactured for eachspecific application. Coupling of magnetic flux into the work piececontaining the metal core has been limited by the physical constraintsof winding a coil about a work piece. The term "work piece" used hereinincludes the plastic article or component with the fusible metal coretherein. Most of these articles or components have unusual geometriesand are surrounded by plastic material which inhibits placing the coilconductors in close proximity with the fusible metal core.

SUMMARY OF THE INVENTION

It has been found that by utilizing a magnetic core made of a materialwith a high permeability, and utilizing a coil around a portion of themagnetic core, then a far better coupling between the coil and themagnetic core is obtained than when a coil is used with a non-magneticwork piece. A single coil may be employed about a magnetic core, and thecore produces a magnetic circuit that may be applied to many differentshapes of work pieces thus eliminating the need for a separate coildesign for each different shape of work piece. Furthermore, magneticnozzles of different shapes may be placed in a gap in the magnetic coreto position or adjust flux lines to suit different shapes of workpieces. The work piece is positioned between the nozzles to provide easyaccess as the work piece does not have to be surrounded by a tightlycoupled coil. The nozzles may be hinged or removable (if required) sothat robotic insertion of the work piece into the gap between thenozzles may be achieved thus making the insertion and removal of thearticle or component simpler.

The present invention provides a process of melting metal cores frommolded plastic articles comprising the steps of, positioning at leastone plastic article in a gap between two magnetic nozzles attached to ahigh permeability magnetic core so that the article remains within amagnetic circuit of the magnetic core, the nozzles shaped to direct andposition a desired pattern of flux lines for a specific shape of plasticarticle, and inducing eddy currents in a metal core of a molded plasticarticle to heat and melt the metal core.

The present invention also provides an induction heating system formelting of metal cores from molded plastic articles comprising, a highpermeability magnetic core, an induction heating coil about at least aportion of the magnetic core to produce flux circulation in a magneticcircuit formed by the magnetic core, and two magnetic nozzles attachedto the magnetic core, with a gap therebetween for insertion of aspecific shape of plastic article, the magnetic nozzles shaped to directand position a desired pattern of flux lines to heat and melt the metalcore of the plastic article.

BRIEF DESCRIPTION OF THE DRAWINGS

In drawings which illustrate the embodiments of the invention:

FIG. 1 is an isometric view of a magnetic core according to oneembodiment of the present invention showing a work piece positioned in agap between two magnetic nozzles.

FIG. 2 is a detailed sectional view through a gap in a magnetic coreshowing two nozzles contoured to fit a specific work piece.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, FIG. 1 shows a magnetic core 10positioned in a tank 12 filled with a heating liquid, preferably oil.The liquid in the tank 12 is heated by means not shown herein andpreferably kept at a temperature slightly above the melting temperatureof the metal core to be melted from within the plastic article. Themagnetic core 10 has high permeability and is preferably made oflaminated magnetic steel or, alternatively, of machinable ferrite toensure flux circulation around the magnetic circuit formed by themagnetic core 10. An induction heating coil 14 is shown wrapped aroundone portion of the magnetic core 10 to couple the flux from the coil 14to the magnetic core 10 and direct flux lines about the magnetic core10. A gap 16 is provided at the top of the magnetic core 10 which hastwo magnetic nozzles 18, one on each side of the gap 16 attached to theends of the magnetic core 10. The space between the nozzles 18 containsthe work piece 20 and provision is made to retain the work piece 20within the gap 16 by a non-magnetic work holder 22 (preferably made ofplastic). The magnetic nozzles 18 are sometimes referred to as fluxconcentrators to the extent that they are shaped to achieve the desiredflux pattern passing through the work piece 20. In effect, the magneticnozzles direct and position the flux lines so the eddy current heatingof the metal core in the work piece 20 is as efficient as possible, andlittle or no loss of heat by hysteresis occurs. Smaller work pieceswould require the nozzles 18 to reduce the area from the ends of thecore 10 to match the size of the work piece 20. The flux nozzles 18 arepreferably made of machinable ferrite, laminated magnetic steel or othersuitable high permeability material. A non-magnetic shim 24 is shownbetween one end of the core 10 and the flux nozzle 18. Such a shim 24which may be made of laminated aluminum or high temperaturethermoplastic material can be used to adjust inductance within themagnetic circuit, and can be located at one or both ends of the core 10.

The air gap 16 between the nozzles 18 linearizes the system andvirtually eliminates hysteresis loss in the system. The laminatedmagnetic steel or machinable ferrite, from which the core 10 is made, isresistant to the flow of eddy currents, thus any eddy current losses inthe magnetic circuit is minimal and any losses which do occur serve toheat the oil tank 12.

FIG. 2 illustrates a particular shape of work piece 20 with nozzles 18contoured to fit the work piece 20. In this instance the nozzles 18 aremade to be hinged or removable from the core 10, but have good contactwith the ends of the core 10, thus different shaped nozzles 18 may beplaced in the gap between the ends of the core loop 10 to suit differentshapes of work pieces 20.

The work piece 20 is supported by work holders 22 made of non-magneticmaterial such as thermoplastics, and the flux nozzle profile is designedto melt the cores 26 from the plastic article forming the work piece 20.The cores 26 melt from the bottom up so the molten metal drops to thebottom of the oil tank. The location of the work piece in the air gap 16is not critical. The removable nozzles 18 are tailored to the work pieceand are easily changed when a different shape of work piece is to beprocessed.

In situations where heating is not desired, for instance, where aspecific component of a work piece is not to be heated, then copper orother suitable shield material is used at a specific location on thework piece 20 or in the nozzle itself to prevent flux lines passingthrough that specific location and heating the area.

In operation, the work piece 20 is placed between the nozzles 18, eitherby an automatic robot system or other suitable insertion system, poweris turned on to the heating coil 14 so that flux circulates around thecore loop 10 and through the work piece 20. Induction heating or eddycurrent heating as it is sometimes called does not directly heat theplastic material, but heats the metal core. General heating times are inthe order of thirty seconds to one minute, the metal core then melts anddrains from the plastic article 20. The heating times can be varied bythe power input to the electrical heating coil 14 and also by changingthe frequency of the electrical power. Because the heating occursquickly, and by induction heating the plastic material, which is notaffected by the induction heating, does not have a chance to heat up,thus the temperature of the metal core is quickly heated and drains awaybefore the temperature of the plastic work piece reaches the meltingtemperature of the metal core or indeed the temperature of the liquidwithin the bath 12.

The temperature from the eddy current heating can be varied by varyingthe number of coils in the heating coil 14 to change the inductance. Theair gap 16 between the nozzles 18 can be changed or, alternatively, agap may be provided between the nozzles 18 and the end of the magneticcore 10. Such an adjustment affects the heat output from the magneticcircuit. The nozzle thickness may vary in thickness to five times theflux lines for a specific size or shape of work piece. Use ofnon-magnetic shims, such as laminated aluminum, high temperaturethermoplastic materials or other suitable non-magnetic materials may beused between the nozzles 18 and the ends of the magnetic core 10 toadjust inductance within the magnetic circuit.

Heating times are greatly reduced by utilizing induction heating as ifthe work piece had to be heated in a hot oil bath, then heating timescan be between 10 to 20 minutes. The efficiency of the system isimproved because flux is more efficiently directed to the workpiecerather than wasted in the surrounding environment, thus less current isrequired to achieve the same flux density through the core. This resultsin reduced copper losses (IR losses) occurring in the heating coil.Furthermore, less current is required to generate the same flux due tothe high permeability of the core, which also reduces copper losses inthe heating coil.

Various changes may be made to the embodiments described herein withoutdeparting from the scope of the present invention which is limited onlyby the following claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. An induction heatingsystem for melting metal cores from molded plastic articles comprising:ahigh permeability magnetic core; an induction heating coil about atleast a portion of the magnetic core to produce flux circulation in amagnetic circuit formed by the magnetic core, and two magnetic nozzlesattached to the magnetic core with a gap therebetween for insertion of aspecific shape of plastic article, the magnetic nozzles shaped to directand position a desired pattern of flux lines to heat and melt the metalcore of the plastic article, wherein a nonmagnetic shim is locatedbetween at least one of the two magnetic nozzles and the magnetic coreto adjust inductance within the magnetic circuit.
 2. The systemaccording to claim 1 wherein the non-magnetic shim is made from materialselected from the group consisting of laminated aluminum and hightemperature thermoplastics.
 3. The system according to claim 1 whereinthe magnetic core is contained in a hot liquid bath.
 4. The systemaccording to claim 1 further comprising:non-magnetic means in contactwith the molded plastic article for supporting the article within themagnetic circuit formed by the magnetic core.
 5. An induction heatingsystem for melting metal cores from molded plastic articles comprising:ahigh permeability magnetic core contained in a hot liquid bath; meansincluding an induction heating coil about at least a portion of themagnetic core for producing flux circulation in a magnetic circuitformed by the magnetic core; two shaped magnetic nozzle means fordirecting and positioning a desired pattern of flux lines to heat andmelt the metal core of the plastic article, the magnetic nozzle meansbeing attached to the magnetic core with a gap therebetween forinsertion of a specific shape of plastic article; and non-magnetic meansin contact with the molded plastic article for supporting the articlewithin the magnetic circuit formed by the magnetic core.
 6. The systemaccording to claim 5 wherein the magnetic core is made of laminatedmagnetic steel.
 7. The system according to claim 5 wherein the magneticcore is made of machinable ferrite.
 8. The system according to claim 5,further comprising means for removably attaching the magnetic nozzlemeans to the magnetic core.
 9. The system according to claim 5 whereinthe magnetic nozzle means are made of machinable ferrite.
 10. The systemaccording to claim 8 wherein the magnetic nozzle means are specificallycontoured to fit and provide a desired flux pattern for a particularshape of work piece.